1. Field of the Invention
This invention relates to wireless communications and, more particularly, to a digital transmitter architecture for a wireless communications system.
2. Description of Related Art
The service area of a wireless communications system is partitioned into connected service domains known as cells, where wireless units communicate via radio links with a base station (BS) serving the cell. The base station is coupled to a land network, for example through a Mobile Switching Center (MSC) which is connected to a plurality of base stations dispersed throughout the service area. In the wireless communications industry, service providers are often granted two or more non-contiguous or segregated frequency bands to be used for the wireless transmission and reception of RF communications channels. For example, in the United States, a base station for an “A” band provider for cellular communications receives frequency channels within the A (825-835 MHz), A′ (845-846.5 MHz) and A″ (824-825 MHz) bands, and transmit on frequency channels within the A (870-880 MHz), A′ (890-891.5 MHz) and A″ (869-870 MHz) bands. A base station for a B band provider receives frequency channels within the B (835-845 MHz) and B′ (846.5-849 MHz) frequency bands, and transmits on frequency channels within the B (880-890 MHz) and B′ (891.5-894 MHz) frequency bands. Additionally, a base station for a Personal Communications Systems (PCS) provider may receive frequency channels from wireless units on one or more blocks of the PCS band (1850 MHz-1910 MHz), and transmit on frequency channels on one or more blocks of the PCS band (1930-1990 MHz).
In a typical transmitter architecture, the baseband information signals are digital signals which are provided to signal processing units (SPUs). The SPUs take the baseband digital signals and perform encoding, error detection processing, bit interleaving and digital in-phase (I) and quadrature (Q) modulation on the digital signals. The resulting digital (I/Q) modulated intermediate frequency (IF) signals are summed together and provided to a digital to analog converter (DAC). The DAC converts the digital IF signals into analog IF signals which are frequency upconverted using analog mixers into analog radio frequency (RF) signals for transmission at the appropriate frequency bands. A common transmitter for the simultaneous processing and transmission of signals within the non-contiguous frequency bands could reduce system hardware costs.
Currently, the transmission systems have an analog low pass filter to remove periodic images of the analog IF signals produced at higher frequencies by the DAC as a result of the digital to analog conversion process. When the digital IF spectrum is converted into the analog domain by a DAC at a conversion rate, the signal bandwidth rotates, or folds, periodically at multiples of one-half the conversion frequency. As such, replica and mirror images of the signal bandwidth are periodically repeated at frequency intervals corresponding to the conversion rate of the DAC.
For example, FIG. 1 shows the analog filter requirements for a DAC where the fundamental analog output frequency f0=10 MHz, which corresponds to the digital input frequency of 10 MHz, and the conversion rate is 30 megawords per second (Mwps) or 30 megasamples per second (Msps), which is equivalent to a conversion frequency of 30 MHz, for the top graph and a conversion rate of 60 Mwps or 60 Msps, which is equivalent to a conversion frequency of 60 MHz, for the bottom graph. The DAC converts the 10 MHz digital signal to a fundamental analog signal image 12a at 10 MHz. With a conversion rate of 30 MHz, the DAC outputs a mirror image 14a at 20 MHz of the fundamental analog signal 12a. Because the signal bandwidth and mirror images of the signal bandwidth are periodically repeated at frequency intervals corresponding to the conversion rate, a replica image 12b of the analog signal is produced at 40 MHz, and a mirror image 14b is produced at 50 MHz along with a replica image 12c at 70 MHz and a mirror image 14c at 80 MHz. For a DAC with a conversion rate of 30 MHz, current design practices use an analog low pass filter (LPF) to remove any images 12b-c and 14a-c and so on from the output of the DAC, leaving the fundamental 10 MHz analog signal image for upconversion to the appropriate RF frequency for transmission.
With a conversion rate of 60 MHz (or 60 Msps or 60 Mwps), the DAC outputs the fundamental 10 MHz analog signal image 16a along with a mirror image 18a at 50 MHz. Because the signal bandwidth and mirror images of the signal bandwidth are periodically repeated at frequency intervals corresponding to the conversion rate, a replica image 16b of the analog signal is produced at 70 MHz, and a mirror image 18b is produced at 110 MHz. As described above, current design practices use an analog low pass filter (LPF) to remove any images 16b, 18a, 18b and so on from the output of the DAC, leaving the fundamental 10 MHz analog signal image for upconversion to the appropriate RF frequency for transmission.
As technology improves, the conversion rate for DACs increases. Current transmitter architectures, however, do not take advantage of the potential cost savings and flexibility which could be provided by the DACs.